This is a request to purchase an electron microscope and to maintain it for an initial three-year period. The microscope is intended to replace an existing 12-year-old one that is nearing the end of its useful life. It will serve the varied needs for electron microscopy of a Core Users' Group of four, who will account for about 75% of instrument time. The microscope will be used in four ongoing studies: in an investigation of the detailed mechanism of assembly of cytoplasmic microtubles and of microtubule-neurofilament interaction; in isolation and characterization of subcellular organelles and in studies of membrane structure involving reconstituted membranes of known composition; in investigation, in phage T4 and in plants, of the mechanisms of DNA replication and recombination, their interrelation and the interaction of participating components; in a series of studies of mechanism of DNA synthesis in plasmids of E. coli. Most of this work might be described as medium resolution analytical microscopy. High resolution studies are also projected in the course of work on microtubule-neurofilament interaction and in the membrane reconstitution studies. The instrument is intended as a facility for multiple users, with emphasis being placed on readiness of access and ease and reliabiliity of use. Existing rooms will be employed to install the microscope. Existing arrangements for administration of use will be adapted from the present electron microscope to the new one. In addition to the Core Users' Group, a Potential Users' Group of seven has been identified. Two of these users have immediate use for the instrument as parts of NSF-funded projects. The other five have had use for electron microscopy in the past and anticipate such use in the future.